[TenTec] OT: Toroid ratings / selection (was TenTec 228 ATU)

[Marinus Loewensteijn]

>> Found a construction description of a 1:9 unun with a FT114-61 toroid that 
>> spans several bands. Will try this setup with a low pass L-C matching network 
>> since it will have very low losses.

>But not zero losses. That toroid will survive 100 watts, but not 1.5 KW.

The TenTec 228 ATU was sold as a 200 Watt ATU. If matching 50 Ohm coax to
50 Ohm coax (I think this is called conjugated match) without a reactance with 
the components then the loss is around the 25% mark. I feel 200 Watt is
very optimistic, would rate it more like 50 - 100 Watt. The 100 Watt capability
of the FT114-61 can be lived with since I am going to use it behind the Omni
546C.

>> The backup plan will be to build an S-Match and then use a Guanella 4:1 output
>> unun for matching the lowest impedances. According to some information the 
>> losses in properly build Guanella are low and it can handle up to a 1:5 SWR.

>Switching in a low to medium Z output transformer can take care of low
>feed Z antennas, nothing new there, is especially handy when feeding
>short loaded verticals.

>Maybe for your loads, you want an output transformer wound to have a tap
>for 12.5 ohms (4:1 from the 50 ohm winding),  no transformer for 50 ohm
>load, and 1:4 for the 200 ohm load region. It would have 3 or four
>winding. I might be lazy and make it with four windings in quad filar,
>just enough turns so that two windings in series gives the right winding
>for 50 ohms. Then the connection between those two windings is the 12.5
>ohm load point. The other two windings in series and connected to the 50
>ohm input gives a 200 ohm load point at the extreme and a 9 * 12.5 or
>112.5 ohm load tap. More winding taps can give more load values to
>minimize the impedance ratio of the tuner and so give it the best chance
>for low loss and broad bandwidth.

This was my first approach however output baluns start to behave strange when
there are reactances and impedances present that are quite some distance 
removed from what it was originally designed for. Losses can get quite high
with resulting overheating of the un-un / balun.

Trouble is that one may have an acceptable SWR at the antenna, e.g. a SWR 
of less than two, but by the time this arrives at the ATU it may well have changed
into something higher / lower due to the transmission line length. Lower is not
the problem, it is the higher that gives the grieve.

>However the minor losses of this
>output transformer may be more than that of an L, Pi, or T match
>covering that load range made with large low loss parts.

The Q has little influence on the efficiency, it is just the T-match design. To get 
broadband matching networks  to function properly with very low impedances 
requires impractical capacitor values.

For a long time I have wonderd about when to use a ferrite and when to use a 
powdered iron wondered in power matching. Have read untold, and often heated, 
discussions on this subject. 

I have made for myself notes on the use of them and I'll relate them here. It may
help others who have been just as confused as I have been.

(The caveat this is my impression and have no scientific proof to back this up.)

Ferrite is selected to to minimize the number of windings required for a given 
inductance. Often the material selected is the one that gives the lowest number 
of windings. In general ferrite toroids span in the balun - unun application 
several octaves more satisfactory use than the powered iron counterparts.

However, due to the ferrite material characteristics, the material with the least 
number of windings has the highest amount of loss. 
When a ferrite toroid overheats then it loses it's permeability and the toroid
is no longer useable. 

When an iron powered toroid is overheated and then cools down it will just
continue to work, it has not been destroyed. Core losses in ferrite are higher
than the core losses in iron powder. In ferrite, if the wrong material is selected,
there can be 1%. Powdered iron is more forgiving and does not run above 0.1%.

If you drop a ferrite then you can just superglue this back together again and
it will work fine. Make the join as thin as possible. An iron powered cannot be 
glued back and is a write off.

Often the wrong ferrite is selected in order to minimize the number of windings
and the result may be that the ferrite material goes into non-linear function
which causes all kinds of distortion in the transmitted signal. However for this
to happen you need high amounts of power and often the toroid overheats 
first due to not being a pure resistive match.

Ferrites are far more unforgiving of high SWR than a powered iron core.

Once again - this is my understanding from what I have read and if I am wrong
then please correct it.  Perhaps it may shed light on the correct use for others
too rather than having one author touting his horn of how wonderfull his idea
is and that nothing else will do.

73, Marinus, ZL2ML
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